I think it is B because it seems like the most valid choice.
Answer:
a = 3.27 m/s²
T = 275 N
Explanation:
Given that:
Mass m₁ = 42.p0 kg
Mass m₂ = 21.0 kg
Consider both masses to be in a whole system, then:
The acceleration can be determined as:

Making acceleration the subject in the above formula;




a = 3.27 m/s²
in the string, the tension is calculated using the formula:



T = 274.68 N
T ≅ 275 N
<span>At an instant when the displacement is equal to a/2,
Potential energy U = 1/2ka(square) where a is displacement.
when a= a/2
U = 1/4ka(square)
U = E/4
Potential Energy = 1/4 Total energy</span>
Answer:
Magnetic field, B = 0.004 mT
Explanation:
It is given that,
Charge, 
Mass of charge particle, 
Speed, 
Acceleration, 
We need to find the minimum magnetic field that would produce such an acceleration. So,

For minimum magnetic field,



B = 0.004 T
or
B = 4 mT
So, the magnetic field produce such an acceleration at 4 mT. Hence, this is the required solution.